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Microplastic pollution constitutes a serious environmental problem that requires more effective scientific research to describe its potential impacts on marine fauna. The interaction between microplastics and marine biota can have significant negative effects through the trophic chain, up to human health. To date, several steps forward have been made in our understanding of this phenomenon; however, large knowledge gaps still exist for several taxa and areas. In particular, the pattern of spatial and temporal distribution of microplastics in marine sediments and their interaction with benthic detritivore species still needs to be addressed. The Mediterranean Sea is one of the most impacted areas of the world, and its biota is deeply affected by microplastic pollution. To investigate the effects of the presence of microplastics in the sediments in this area, the echinoderm Holothuria tubulosa was chosen as a model species, and specimens were collected along the Salento peninsula in Apulia, Southern Italy. This peculiar geographic area extends between two ecoregions of the Mediterranean Sea, the Northern Ionian and the Southern Adriatic seas, characterized by peculiar and distinct currents and submarine topologies, resulting in a complex and dynamic ecosystem affected by seasonal fluctuations that make the Salento peninsula an interesting natural laboratory for predictions of future dispersion events on a wider scale. Microplastics were analyzed by investigating the gut contents of H. tubulosa individuals, and the SEM/EDX method was used to confirm the plastic material extracted. Results revealed microplastics in all the specimens analyzed and with a homogeneous pattern of distribution in time and some differences in space, suggesting that the presence of this anthropogenic material is constant throughout the year and its quantity is only slightly affected by the level of conservation and management strategies characterizing the sampling sites.

Marionia gemmii Almón, Pérez & Caballer, 2018 has been described as a new species from six localities in Ría de Arousa, Galicia (North-Western Spain) and in the Golfe de Cádiz (Southern Spain), both within Atlantic waters. Old and new findings of this species from the North-Western Mediterranean radically expand its distribution and an in-depth study of the images available on the web dates its first documented record in this area back to 2009.

The aeolid nudibranch Caloria elegans (Facelinidae) is quite common in the Mediterranean Sea and eastern Atlantic Ocean and is easily recognized by the presence of a typical black spot at the apical portion of its cerata. Facelina quatrefagesi (Facelinidae) was long considered as a synonym of C. elegans until recently, when it was re-evaluated as a valid species based mainly on rhinophore morphology. In order to definitively assess the status of these aeolid taxa, we employed an integrative taxonomy approach using the nuclear H3 and the two mitochondrial cytochrome oxidase subunit I and 16S markers. The molecular analyses clearly showed that, although morphologically closely related to C. elegans, F. quatrefagesi is a valid species.

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Members of the globally distributed bivalve family Ostreidae (oysters) have a significant role in marine ecosystems and include species of high economic importance. In this work, we report the occurrence of digenean parasites of the families Bucephalidae (Prosorhynchoides sp.) and Monorchiidae (Postmonorchis sp.) in Mediterranean native populations of Ostrea edulis (but not in the introduced Magallana gigas). Molecular detection was based on DNA sequencing of the ribosomal intergenic spacer 2 (ITS2) marker. The importance of detecting the presence of overlooked digenean parasites in Mediterranean oysters is discussed.

Quantitative studies on population dynamics and life history traits of key species are useful to predict changes in the structure and organization of biological communities. In this context, Hermodice carunculata (Pallas, 1766) is a selective scavenger/carnivore polychaete species (known as a fireworm) that, in recent years, has exhibited a northern expansion along the whole basin, including the Italian coasts, and an increasing abundance in its southern areas. Here we report ecological data and fireworm abundances from two shallow stations of the Salento peninsula in the Ionian Sea (Lecce, Italy), characterized by different hydrodynamic exposure levels and structural communities. The observed densities by visual census in the years 2019–2021 (up to 18 ind./15m2) document a striking shift compared to the past anecdotal reports of the rare occurrence of fireworms along the studied area. Additionally, their abundance seems to be driven by the substrate coverage; in fact, a higher density of fireworms was observed where the biodiversity is richer. Results from this study will serve as a baseline reference for future investigation of the invasiveness potential of a species that can act as a biological marker of ocean warming.

Nudibranchs have a mostly carnivorous diet, and they prey on a wide variety of other animal taxa. Many species, mainly belonging to the Cladobranchia suborder, feed on cnidarians, including member of the class Hydrozoa. Several hydrozoan species display a symbiotic lifestyle, being associated with other benthic invertebrates, including for instance bryozoans, corals, octocorals, and sponges. In our knowledge, no record of nudibranch predation on symbiotic hydrozoans has been reported so far, possibly thanks to the protective action by the host towards its symbiotic hydrozoan. Here, we show the unexpected case of a nudibranch belonging to the recently described species Sakuraeolis marhe (Fernández-Simón and Moles, 2023) feeding on Zanclea sp. 2, a hydrozoan associated with the cheilostome bryozoan Celleporaria sp. This trophic association is confirmed by the presence and storage of the nematocysts into the nudibranch cnidosacs. Moreover, the nudibranch appears to selectively store mostly a single type of nematocyst, that is large size stenotele. The observation here reported represents the first well-documented record of a nudibranch feeding on a symbiotic hydrozoan and the first confirmed case of predation on Zanclea polyps. Moreover, we provide additional genetic information and the first description of the internal anatomy of S. marhe.

The introduction rate of alien species in the Mediterranean Sea is rapidly growing, and their taxonomical identification is increasingly challenging. This uncertain identification often leads to an incorrect estimation of the number of alien species, their route of introduction, and their potential negative effects. This is particularly true for some bivalves, which are characterized by high variation in their shells, resulting in uncertain morphological identification. This is the case for two alien bivalves, i.e., an Isognomonidae and a Malleidae species, both characterized by confused historical colonization records in the Mediterranean Sea, misidentifications, and controversial and changing nomenclatures that have insofar negatively affected our knowledge on their geographical distributions. In this respect, molecular approaches provide a strategy that is especially useful when traditional taxonomy fails, and DNA barcoding is a powerful and well-known tool to obtain reliable identifications through efficient molecular markers. In this work, we used the 16S rRNA marker to assess the preliminary identification of Isognomon sp. and Malleus sp. specimens from different localities in the Southern Mediterranean Sea. Bayesian inference (BI) and maximum likelihood (ML) methods were applied to test the monophyly of the phylogenetic linages and to clarify their taxonomic positions, allowing a complete overview of the colonization and spreading of these two alien bivalves in the Mediterranean Sea. In particular, the Isognomon sp. specimens were identified as the Atlantic I. bicolor, highlighting that previously suggested invasive migration patterns, (i.e., the Lessepsian migration), must be reconsidered with stronger critical attention in light of currently occurring global changes.

Mediterranean marine fauna is constantly changing due to the entry of non-indigenous (NI) species and the loss of endemic biodiversity. In this framework, it is very important to monitor this constant change and investigate possible new pathways of dispersion. Marinas and ports are considered key stations to detect and study some important ecological aspects, such as NI and invasive species, the effects of climate change, and pollution. Here, we reported the case of a group of NI species that presumably reached the Mediterranean Sea together, each of them being ecologically associated with one another. The bryozoan Amathia verticillate and the sea slugs Favorinus ghanensis and Polycerella emertoni were found in the shallow waters of Fezzano’s marina in the gulf of La Spezia (Ligurian Sea, Mediterranean Sea). Molecular analyses were carried out to exclude cryptic diversity and to investigate the phylogenetic relationships occurring between closely related taxa. The spreading of these two NI sea slugs into the Mediterranean Sea was confirmed and the first record of P. emertoni from the Ligurian Sea reported. These findings shed some light on the poorly known ecology of these species that could be useful for future monitoring and conservation strategies.

We report the discovery of the Indo-Pacific sacoglossan Elysia nealae Ostergaard, 1955 from northeastern Sardinia (Central Tyrrhenian Sea). This is the first record of this species in the western Mediterranean Sea and only the second for the whole of the Mediterranean Sea following a report from Cape Epanomi, Greece. We discuss the identification of this species as well as the expansion of its geographical range. Data on the ecology and behavior of E. nealae leads us to hypothesize that the increase in the Mediterranean’s water temperature due to climate change has favored this non-native species and contributed to its expanded distribution.

The Mediterranean Sea is a hot spot for marine biodiversity, and this is particularly evident taking into consideration the diversity observed in many animal groups, among them the Molluscs. In the last decade, several works have revealed a high rate of cryptic diversity characterizing the Molluscan fauna of the Mediterranean Sea and an increasing number of endemic and/or new species inhabiting this semi-enclosed basin have been recorded or described. The DNA-barcoding method is considered an essential step in the integrative taxonomy applications, to unravel cryptic diversity and for species identification. Here we report the case of DNA-barcoding technique applied to identify a nudibranch (Heterobranchia) collected from the Adriatic Sea, in the Bay of Kotor (Montenegro), for which a standard morphological identification was not possible. Mediterranean specimen belonging to Pruvotfolia pselliotes (Labbé, 1923) is for the first time molecularly identified and its COI DNA sequence compared with the one of an individual collected from the type locality. In addition, this is the first verified report of this species from the Adriatic Sea. Finally, the potential of using DNA-barcoding is here discussed, together with the habitat and the geographical distribution of this uncommon species.

The mollusc fauna of the Mediterranean Sea is still considered as the best-known marine mollusc fauna in the world. The previous modern checklists of marine Mollusca were produced by joint teams of amateurs and professionals. During the last years the Italian Society of Malacology (Società Italiana di Malacologia – S.I.M.) maintained an updated version of the Mediterranean checklist, that served as the backbone for the development of the new Italian checklist. According to the current version (updated on April 1st, 2021), 1,777 recognised species of marine molluscs are present in the Italian Economic Exclusive Zone, including also the Tyrrhenian coasts of Corsica and the continental shelf of the Maltese archipelago. The new checklist shows an increase of 17% of the species reported in the 1995 Checklist. This is largely (yet not solely) due to the new wave of studies based on Integrative Taxonomy approaches. A total of 135 species (7.6%) are strictly endemic to the Italian waters; 44 species (2.5%) are alien and correspond to the 28% of the Mediterranean alien marine molluscs. All eight extant molluscan classes are represented. The families represented in the Italian fauna are 307, an increase of 14.6% from the first checklist, partly due to new records and partly to new phylogenetic systematics. Compared with the whole Mediterranean malacofauna, the Italian component represents 71% in species and 61% in families, which makes it a very remarkable part of the Mediterranean fauna.

Understanding the impacts of microplastics on living organisms in aquatic habitats is one of the hottest research topics worldwide. Despite increased attention, investigating microplastics in underwater environments remains a problematic task, due to the ubiquitous occurrence of microplastic, its multiple modes of interactions with the biota, and to the diversity of the synthetic organic polymers composing microplastics in the field. Several studies on microplastics focused on marine invertebrates, but to date, the benthic sea slugs (Mollusca, Gastropoda, Heterobranchia) were not yet investigated. Sea slugs are known to live on the organisms on which they feed on or to snack while gliding over the sea floor, but also as users of exogenous molecules or materials not only for nutrition. Therefore, they may represent a potential biological model to explore new modes of transformation and/or management of plastic, so far considered to be a non-biodegradable polymer. In this study we analysed the stomachal content of Bursatella leachii, an aplysiid heterobranch living in the Mar Piccolo, a highly polluted coastal basin near Taranto, in the northern part of the Ionian Sea. Microplastics were found in the stomachs of all the six sampled specimens, and SEM/EDX analyses were carried out to characterize the plastic debris. The SEM images and EDX spectra gathered here should be regarded as a baseline reference database for future investigations on marine Heterobranchia and their interactions with microplastics.

The genus Nemesis Furfaro & Mariottini, 2021, was recently introduced for an independent lineage of aeolid nudibranchs, and Dondice banyulensis Portmann & Sandmeier, 1960, established as its type species. Anyway, the presence of a senior homonym, Nemesis Risso, 1826, was evidently missed. In fact, in 1826, Risso established this genus for a group of Copepoda (Arthropoda, Crustacea) and according to the Principle of Priority (ICZN) only the senior homonym may be used as a valid name. Therefore, a new replacement name is here proposed. Furthermore, the genus name Nanuca Er. Marcus, 1957, has priority over Dondice Er. Marcus, 1958 and consequently, the species in this clade should be classified under Nanuca, mostly as new combinations.

Mediterranean marine biodiversity is still underestimated especially for groups such as nudibranchs. The identification of nudibranchs taxa is challenging because few morphological characters are available and among them chromatic patterns often do not align with species delimitation. Molecular assessments helped unveiling cryptic diversity within nudibranchs and have been mostly based on mitochondrial markers. Fast evolving nuclear markers are much needed to complement phylogenetic and systematic assessments at the species and genus levels. Here, we assess the utility of the nuclear Internal Transcribed Spacer 2 (ITS2) to delimit species in the eolid nudibranchs using both primary and secondary structures. Comparisons between the variation observed at the ITS2 and at the two commonly used mitochondrial markers (COI and 16S) on 14 eolid taxa from 10 genera demonstrate the ability of ITS2 to detect congeneric, closely related, species. While ITS2 has been fruitfully used in several other mollusc taxa, this study represents the first application of this nuclear marker in nudibranchs.

Integrative taxonomy is an evolving field of multidisciplinary studies often utilised to elucidate phylogenetic reconstructions that were poorly understood in the past. The systematics of many taxa have been resolved by combining data from different research approaches, i.e., molecular, ecological, behavioural, morphological and chemical. Regarding molecular analysis, there is currently a search for new genetic markers that could be diagnostic at different taxonomic levels and that can be added to the canonical ones. In marine Heterobranchia, the most widely used mitochondrial markers, COI and 16S, are usually analysed by comparing the primary sequence. The 16S rRNA molecule can be folded into a 2D secondary structure that has been poorly exploited in the past study of heterobranchs, despite 2D molecular analyses being sources of possible diagnostic characters. Comparison of the results from the phylogenetic analyses of a concatenated (the nuclear H3 and the mitochondrial COI and 16S markers) dataset (including 30 species belonging to eight accepted genera) and from the 2D folding structure analyses of the 16S rRNA from the type species of the genera investigated demonstrated the diagnostic power of this RNA molecule to reveal the systematics of four genera belonging to the family Myrrhinidae (Gastropoda, Heterobranchia). The “molecular morphological” approach to the 16S rRNA revealed to be a powerful tool to delimit at both species and genus taxonomic levels and to be a useful way of recovering information that is usually lost in phylogenetic analyses. While the validity of the genera Godiva, Hermissenda and Phyllodesmium are confirmed, a new genus is necessary and introduced for Dondice banyulensis, Nemesis gen. nov. and the monospecific genus Nanuca is here synonymised with Dondice, with Nanuca sebastiani transferred into Dondice as Dondice sebastiani comb. nov.

During the last twenty years, several studies have demonstrated the primary role of the lithological properties of hard substrates in conditioning the diversity and structure of marine sessile communities (aka bio-mineralogy). This research was dedicated to testing whether different substrates (limestones and granites) influence higher levels of the trophic net. For this purpose, the diversity and abundance of heterobranch molluscs were used to test the hypothesis that their distribution could mirror their prey. The study was conducted in the ‘Tavolara-Punta Coda Cavallo’ Marine Protected Area (NE Sardinia, Mediterranean Sea), where substrates of different lithology co-occur in a restricted area. Diversity and abundance of heterobranchs were investigated by photographic sampling (3300 studied images) in twelve sites (six limestones and six granites), between 35 and 50 m depth, demonstrating clear preferences of several species for a specific substrate, while others are ubiquitarian. In general, diversity and abundance were higher on limestones due to the exclusive presence of Sacoglossa coupled with a more significant number of Cladobranchia and Doridina; on the other hand, Umbraculida were strongly granite-selected. In some cases, the different affinity of each species was clarified by the distribution of their food sources, but, in others, it is possible to hypothesise that the structure of the whole benthic community, affected by the substrate characteristics, may drive the heterobranchs' distribution.

Species delimitation is particularly challenging in Mediterranean nudibranchs because morphological distinction between closely related species is subtle and several cryptic species have been discovered. In this study, we apply an integrative approach to investigate taxonomic distinction of Flabellina cavolini and F.gaditana and assess their genetic and morphologic distinction. We combined a comprehensive phylogeographic survey from across their distribution range and compared genetic variation at mitochondrial and nuclear markers with the variation in colour pattern of cerata that is used to discriminate these two species. Phylogenetic, phylogeographic and species delimitation analyses consistently identify two evolutionary and taxonomic units that do not match with the morphospecies, demonstrating that Flabellina cavolini and F.gaditana are indeed two cryptic, closely related, species and that colour pattern of cerata is not a valid diagnostic character. This study emphasizes how the comparison of range-wide patterns of phylogenetic and morphological variation, with the inclusion of samples from the type localities, is essential for species delimitation of Mediterranean nudibranchs. Moreover, this study demonstrates the occurrence of Flabellina gaditana in the Mediterranean Sea, allowing to extend the known distribution range of the species, that was previously restricted to the eastern Atlantic Ocean, as well as increasing our inventory of Mediterranean nudibranch diversity.

The Mediterranean Sea diversity is still far from being fully disclosed. Marine Heterobranchia are one of the most paradigmatic species-rich groups, with many recent systematic studies revealing the high density of new, cryptic, and endemic species occurring in the Mediterranean basin. In this study, sea slug Doriopsilla areolata, which was considered until today one of the most widespread nudibranchs worldwide, was investigated using a molecular approach to compare Mediterranean and Atlantic populations for the first time. The molecular analyses involved three different molecular markers, the two mitochondrial COI and 16S, and the nuclear H3 gene. The results revealed a complex of species within D. areolata that indeed consists of three potentially species, two of which are endemic to the Mediterranean Sea: Doriopsilla areolata, which is distributed in the Adriatic Sea (the type locality of the former species), D. rarispinosa, which occurs in the Western Mediterranean basin and along the Tunisian coast, and one additional Atlantic species here provisionally defined as Doriopsilla sp. 1. This study helps to unveil another case of cryptic diversity within Mediterranean Heterobranchia and to increase the knowledge on Doriopsilla genus diversity.

The diversity of Mediterranean nudibranchs has yet to be thoroughly studied: new species are constantly described, and molecular approaches have revealed some cryptic species. A new facelinid species has been discovered based on specimens collected from the Tyrrhenian Sea (Mediterranean Sea). Integrative results of molecular analyses and of anatomical investigations support the description of Dondice trainitoi sp. nov. The characteristic chromatic body pattern and the black epithelium covering the masticatory jaws allow an unambiguous identification of the new taxon. Preliminary phylogenetic analyses based on multi-locus molecular markers (nuclear H3 gene and mitochondrial markers 16S rDNA and COI) surprisingly revealed paraphyly of the genus Dondice and the need of further studies including more taxa assigned to the currently accepted family Facelinidae. Furthermore, following an integrative taxonomy approach, considerations on the ecological behaviour characterizing most of the species involved in this study provide useful insights for understanding the evolutionary history of this facelinid group.

The continuing exploration of the Mediterranean deep sea reveals that a complete census of the biodiversity of one of the most studied marine areas in the world is yet to be fully accomplished. A tritoniid nudibranch new to science is described upon material recovered off the Montenegro margin, Adriatic Sea, associated with the alcyonacean Primnoidae Callogorgia verticillata (Pallas, 1766). Here, a coral forest dominated by C. verticillata, together with other alcyonaceans, antipatharians and scleractinians, represents the habitat of the new species. As documented by Remotely Operated Vehicle (ROV) observations, the coral forest covers an area of 600 m2 at 420–426 m depth, with dense growth of C. verticillata colonies (1.34 ± 0.08 colonies m−2). As many as 66 nudibranch specimens have been observed on C. verticillata. Tritonia callogorgiae sp. nov. (Gastropoda, Nudibranchia, Tritoniidae) is up to 12 cm long, yellow to orange/reddish in colour with white papillae all over the body. It has a velum with six appendages (three per side), and 4–5 pairs of dichotomous gills. Internally, the species is distinguished from other tritoniids for the absence of stomach plates and a distinctive radula with formula 50 × 110.1.1.1.108. It represents the first case of Tritonia species living in the deep Mediterranean Sea and the only one associated to C. verticillata. Stomach content documents that Tritonia callogorgiae sp. nov. feeds on C. verticillata. Further visual ROV records from other sectors of the Mediterranean Sea indicate that the distribution of this new described species is almost basin-wide. The discovery of another undescribed member of the benthic megafauna related to deep-sea coral grounds further highlights the need for conservation of such habitats.

The geographic range of Piseinotecus soussi Tamsouri, Carmona, Moukrim & Cervera, 2014 is here updated, since it was found in two new localities along the Tyrrhenian coast of the Mediterranean Sea. This species was previously recorded in Mediterranean basin only from the Ionian Sea (Apulia). With this paper, the range of distribution of this aeolid sea slug is considerably extended. Furthermore, to confirm the identity of specimens recorded, a DNA Barcoding approach was carried out using the mitochondrial Cytochrome c oxidase subunit I (COI) as it is the mostly used barcoder marker in Nudibranchs.

‘Cryptic’ species are an emerging biological problem that is broadly discussed in the present study. Recently, a cryptic species definition was suggested for those species which manifest low morphological, but considerable genetic, disparity. As a case study we present unique material from a charismatic group of nudibranch molluscs of the genus Trinchesia from European waters to reveal three new species and demonstrate that they show a dual nature: on one hand, they can be considered a ‘cryptic’ species complex due to their overall similarity, but on the other hand, stable morphological differences as well as molecular differences are demonstrated for every species in that complex. Thus, this species complex can equally be named ‘cryptic’, ‘pseudocryptic’ or ‘non-cryptic’. We also present evidence for an extremely rapid speciation rate in this species complex and link the species problem with epigenetics. Available metazoan-wide data, which are broadly discussed in the present study, show the unsuitability of a ‘cryptic’ species concept because the degree of crypticity represents a continuum when a finer multilevel morphological and molecular scale is applied to uncover more narrowly defined species making the ‘cryptic’ addition to ‘species’ redundant. Morphological and molecular methods should be applied in concordance to form a fine-scale multilevel taxonomic framework, and not necessarily implying only an a posteriori transformation of exclusively molecular-based ‘cryptic’ species into morphologically-defined ‘pseudocryptic’ ones. Implications of the present study have importance for many fields, including conservation biology and fine-scale biodiversity assessments.

The Atlantic-Mediterranean Bursa scrobilator (Linnaeus, 1758) complex is represented by three morphologically distinct forms, two that are extant corresponding to the nominal taxa Murex scrobilator Linnaeus, 1758 and Ranella coriacea Reeve, 1844, and one fossil corresponding to Murex nodosus Borson, 1825. The complex is here reviewed integrating morphological, molecular (mitochondrial COI marker) and paleontological data. The ‘scrobilator’ and ‘coriacea’ forms are proposed as two subspecies: Bursa scrobilator scrobilator (Linnaeus, 1758) with an Atlantic-Mediterranean distribution, and Bursa scrobilator coriacea (Reeve, 1844) with a West African range. The evolutionary relationships of the two recent subspecies with their most probable ancestor, the Mediterranean Miocene fossil B. nodosa, are also discussed.

The Salento peninsula is a portion of the Italian mainland separating two distinct Mediterranean basins, the Ionian and the Adriatic seas. Several authors have studied the marine Heterobranchia (Mollusca, Gastropoda) fauna composition living in the Ionian Sea, but to date further knowledge regarding this interesting group of mollusks is still needed. Recent studies have corroborated the peculiarity of the Mediterranean Sea showing high levels of endemism and cryptic diversity. On the other hand, marine sea slugs have been revealed to be important indicators of the marine ecosystem’s health, due to their species-specific diet that consist of a vast variety of sessile and benthic invertebrates. A baseline study of the marine Heterobranchia diversity is therefore a necessary step to reveal the hidden diversity and to monitor the possible presence of alien species. The present study shows results from approximately 600 scientific dives carried out during a nine-year period in all of the main submarine habitats of the studied area, while accounting for the marine Heterobranchia from both the Ionian and Adriatic Seas. With this contribution, the list of marine Heterobranchia inhabiting the Salento Peninsula rises to 160. Furthermore, it also reports, for the first time, the presence of one alien species and three new records for Italian waters. Ecological notes and geographical distribution for each added species are provided together with animal iconography, consisting mainly of in situ photographs, for species identification.

A species of nudibranch new to science was recorded on the artificial substratum of the Asia wreck in the Central Tyrrhenian Sea at 30 m depth. Dondice sp. nov. A is diagnosed by both morphological and molecular analysis. The anatomical study involved diagnostic features mostly used in nudibranchs, like the buccal apparatus and the reproductive system, while the molecular investigations have used the mitochondrial 16S and COI and the nuclear H3 molecular markers. Furthermore, the phylogenetic assessment of the Dondice genus and the two mostly related genera Phyllodesmium and Godiva was also performed resolving some historical controversies.

The first record from Sicily of the introduced facelinid nudibranch Godiva quadricolor allowed the detection of trophic relationships with the polycerid Polycera hedgpethi, another non-native nudibranch, and with two bryozoan species, namely the naturalized Cheilostomatida Bugula neritina and the cryptogenic Ctenostomatida Amathia verticillata. The settlement of both nudibranchs was presumably promoted by a trophic shift of P. hedgpethi from the natural prey B. neritina towards the largely available and not exploited A. verticillata. This short food web, without evident links with native fauna and having G. quadricolor as the top predator, is described. A DNA barcoding approach was used to confirm the identity of this facelinid species and to explore the possible genetic divergence occurring among the samples analysed.

Recent molecular studies revealed high level of endemism and numerous cryptic species within opisthobranchs, with Mediterranean taxa clearly understudied. Here we used genetic data from both mitochondrial and nuclear gene fragments as well as morphological data from taxonomically relevant characters to investigate the phylogenetic relationships and systematics of Mediterranean taxa of the Flabellinidae and Piseinotecidae families. Phylogenetic analyses based on Bayesian and Maximum-Likelihood methods indicate that Flabellinidae and Pisenotecidae taxa and species within the genera Flabellina, Calmella and Piseinotecus do not form monophyletic clades. These results are supported by our morphological analyses which allowed the re-evaluation of the triseriate radula condition in Pisenotecidae and Calmella taxa and their inclusion in the genus Flabellina as Flabellina gaditana comb. nov. (synonym of F. confusa), Flabellina gabinierei comb. nov. and Flabellina cavolini comb. nov. Species delimitation and barcoding gap analyses allowed uncovering cryptic species within Flabellina gracilis (Alder and Hancock, 1844), F. trophina (Bergh, 1890), F. verrucosa (M. Sars, 1829) and F. ischitana Hirano and Thompson, 1990, the latter with an Atlantic form which is under description. This study corroborates the relevance of combining molecular and morphological data from multiple populations and species in the assessment of nudibranch diversity and classification.

The southernmost record of the Mediterranean nudibranch Felimida elegantula (Philippi, 1844) is reported here. A single specimen, with a colour pattern that hampered an easy morphological identification, was collected at the Kerkennah Islands (Tunisia). DNA-barcoding analysis (with the cytochrome c oxidase subunit I genetic marker) unambiguously confirmed its identification, and radular and anatomical features conformed with those previously described. With this record, the chromatic polymorphism of F. elegantula is demonstrated to be extremely large and the known range of the species is extended to the African coast of Tunisia.

The nudibranch Tritonia nilsodhneri, usually feeding on a variety of gorgoniacean species, is known from different localities of the eastern Atlantic Ocean and the Mediterranean Sea. Knowledge of the host preferences of the Mediterranean populations is still scarce. Few records of this nudibranch have been reported from the eastern Mediterranean basin. With this report, the occurrence of T. nilsodhneri within the Mediterranean basin is extended to the Adriatic Sea. Furthermore, the list of the host species associated to the Mediterranean populations for feeding habits is increased from two up to five. Mediterranean specimens of T. nilsodhneri were observed for the first time feeding and spawning on Leptogorgia sarmentosa, Eunicella cavolini and E. labiata. Finally, these last two Gorgoniidae species are also reported here as a new host species for T. nilsodhneri., Morski puž golać, Tritonia nilsodhneri, poznat je s različitih lokaliteta istočnog Atlantskog oceana i Sredozemnog mora, a obično se hrani raznim vrstama gorgonija. Podatci o domaćinima za ovu vrstu u Sredozemlju su još uvijek rijetki. Postoji nekoliko zapisa o ovom pužu u istočnom dijelu Sredozemlja. Ovim radom zabilježena je nazočnost vrste T. nilsodhneri u Jadranskom moru. Nadalje, popis vrsta domaćina koji su povezani sa hranjenjem i mriještenjem sredozemne populacije se povećao sa dva na pet. Sredozemni primjerci T. nilsodheri su po prvi put zabilježeni tijekom hranjenja i mriješćenja na vrstama Leptogorgia sarmentosa, Eunicella cavolini i E. labiata. Ove posljednje dvije spomenute vrste Gorgonida su u ovom radu po prvi put zabilježene kao nova vrsta domaćina za T. nilsodheri.